Focus-error detection methods heretofore used for optical storage applications generally employ a knife edge, an astigmatic lens or a critical angle prism. These techniques require very critical alignment of these optical elements and of a segmented photodetector.
Published European patent application EP 0164687 discloses a detection technique wherein a laser beam reflected from an optical disk is directed through an objective lens to a prism that reduces beam width in one dimension by a factor M and delivers an elliptical beam to a knife-edge-type focus error detection system. This application claims that the use of the prism increases the focus error signal by a factor of M compared to the standard knife edge technique without a prism.
The Digest of the Topical Meeting on Optical Data Storage, Oct. 15-17, 1985 at Washington, D.C., includes Paper THCC2-1 by Yamamoto et al. entitled "Design Consideration of Optical Pregroove Dimensions". This paper shows a six-element photodetector to detect the far field spot size variations associated with a focus error.
There is a need for a very sensitive focus-error detection technique that will provide, with relatively few components, a significant enhancement of the focus-error signal and provide a large beam size that only requires an uncritical alignment of a segmented photodetector in one dimension.